1. Field of the Invention
The present invention relates, in an image forming apparatus making use of an electronic photographic system such as a copier, a printer and a facsimile, to an image fixing device for heating and fixing a non-fixed toner image, and particularly to an image fixing device of a belt nip system.
2. Description of the Related Art
FIG. 12 shows an image fixing device proposed by the applicant of the present invention and disclosed in Japanese Patent Laid-Open No. Hei 5-150679 (1993). In the figure, reference numeral 101 designates a heating and fixing roll rotatably arranged. This heating and fixing roll 101 has a cylindrical core 103 formed of metal having a high heat conductivity such as aluminum, and a coating layer 104 formed of an elastic member formed on the surface thereof. The coating layer 104 comprises a primary coating layer 120 formed of HTV (High Temperature Vulcanization) silicon rubber directly coated on the surface of the core 103 and a top coating layer 121 formed of RTV (Room Temperature Vulcanization) silicon rubber coated externally thereof.
A halogen lamp 105 as a heating source is arranged within the core 103. A temperature sensor 106 is arranged on the surface of the heating and fixing roller 101 to measure the temperature of said surface. A temperature controller not shown is actuated by a measurement signal of the temperature sensor 106 to control turn-on and off of the halogen lamp 105 so that the surface temperature of the heating and fixing roll 101 is adjusted. Silicon oil as a release medium is supplied to the surface of the heating and fixing roll 101 by an oil supply device 109 whereby when an image of toner 108 is fixed to a sheet 107, a part of the toner 108 is prevented from being offset to the heating and fixing roll 101.
In the above image fixing device, a pressure roll 125 is pressed against the heating and fixing roll 101 by means of a compression spring 126. Further, a pressure auxiliary roll 129 is pressed toward the heating and fixing roll 101. Rolls 122, 123 and 124 are arranged in the periphery of the pressure roll 125 and the pressure auxiliary roll 129. An endless belt 115 is wound around the rolls 122, 123 and 124, the pressure roll 125 and the pressure auxiliary roll 129.
The pressure roll 125 and the pressure auxiliary roll 129 are pressed against the heating and fixing roll 101 whereby the endless belt 115 comes in contact with the heating and mixing roll 101 to form a nip (a belt nip) for causing the sheet 107 to pass through between the heating and fixing roll 101 and the endless belt 115. The heating and fixing roll 101 and the endless belt 115 rotate as indicated by the arrows respectively and the sheet 107 is carried between the heating and fixing roll 101 and the endless belt 115. When the sheet 107 passes through the belt nip, the non-fixed toner 108 is fixed onto the sheet 107 by the pressure exerting on the belt nip and the heat applied through the heating and fixing roll 101 from the halogen lamp 105.
By employing the construction of the belt nip system as described, the sheet 107 is heated for a period of time corresponding to the width of the belt nip (in the illustrated example, the length of the belt held by the pressure roll 125 and the pressure auxiliary roll 129). This leads to the advantage in that even if the carrying speed of the sheet 107 is increased, it is possible to secure a sufficient fixing time as compared with the case where only the pressure roll and the heating and fixing roll 101 are pressed and the endless belt 115 is not used. Further, if the carrying speed is the same, the heating time in the belt nip system is longer than that in the system not using the endless belt 115 and a large quantity of heat can be given by the toner. Therefore, the belt nip system is suitable particularly for fixing in a color copier which colors a large quantity of toner to a color as desired.
Further, in this image fixing device, the surface of the heating and fixing roll 101 is formed with the coating layer 104 formed of silicon rubber which is an elastic member so that the coating layer 104 receives the pressing force of the pressure roll 125 so as to be locally flexed in a pressing area. That is, since the surface speed of the coating layer 104 is locally high, a fine deviation occurs between the sheet 107 and the coating layer 104 in the pressing area of the pressure roll 125. Further, the toner 108 in a high temperature state tends to be adhered to the coating layer 104, but such a deviation leads to a fine slip in an interface between the toner 108 and the coating layer 104 to prevent the toner 108 from being deposited on the coating layer 104.
Thereby, the sheet 107 is separated from the heating and fixing roll 101 against the adhesive force between the toner 108 and the heating and fixing roll 101. Since the adhesive force of the melted toner to the heating and fixing roll 101 is governed even by the property value of the interface chemical material of both elements, the behavior in which the sheet 107 is stripped varies with kinds of toner and materials of the coating layer 104. However, according to this image fixing device, the sheet 107 can be stripped from the heating and fixing roll 101 even if a stripping means such as a stripper claw is not used (this will be hereinafter called self-stripping). Because of this, a sheet which is easily bent and hard to be stripped or a sheet to which a large amount of toner adhered can be subjected to self-stripping.
Other image fixing devices employing the belt nip system are disclosed in Japanese Patent Laid-Open Nos. Sho 52-69337 (1977), Sho 60-151677 (1985), Sho 60-151681 (1985), Sho 62-14675 (1987) Hei 2-222982 (1990), Hei 2-308287 (1990), Hei 4-50885 (1992) and Hei 4-115279 (1992), and Japanese Utility Model Laid-Open Nos. Sho 60-104852 (1985), Hei 2-30961 (1990), Hei 3-86374 (1991), Hei 3-92661 (1991) Hei 4-50864 (1992).
However, the aforementioned belt nip system image fixing device has the following problems. That is, in the case where the toner 108 of a heat fusion type which is now in the main current is used, the sheet 107 has to be necessarily heated to fuse the toner 108 to the sheet 107. At the time of heating as described, air becomes thermally expanded or water vapor becomes evaporated from a gap between fibers of the sheet 107 or a gap between toner particles. Such air or water vapor is generated when the sheet 107 is heated within the belt nip, is present as foam within the belt nip, concretely between the sheet 107 and the heating and fixing roll 101 or the endless belt 115 till the sheet 107 passes through the belt nip, and is discharged outside at the same time when the sheet 107 has passed through the belt nip. Within the belt nip, an area which is small in pressing force is widely present between the pressing portion by the pressure roll 125 and the pressing portion by the pressure auxiliary roll 129, as shown in FIG. 13, and when the foam is present in this area, a gap is sometimes produced between the heating and fixing roll 101 and the endless belt 115. When such a gap is produced, in the state where the toner 108 on the sheet 107 within the belt nip is not yet fixed, the non-fixed toner 108 becomes moved by the movement of the foam. This leads to a problem in that blur, deviation, disturbance, etc. occurs in the image.
FIG. 14 is a graph showing the results of measurement of the gap produced between the heating and fixing roll 101 and the endless belt 115 in the central portion of the belt nip, that is, in the intermediary between the pressure roll 125 and the pressure auxiliary roll 129 in fixing the image of the non-fixed toner 108 with the device shown in FIG. 12. Sheets used are J sheets of A3 Format manufactured by Fuji Xerox Co., Ltd. The gap is obtained by measuring a spacing between the heating and fixing roll and the endless belt and subtracting the thickness of the sheet from the measured value. As will be apparent from the results shown in FIG. 14, when the temperature of the fixing roll is approximately the environmental temperature, no gap is produced but when the temperature of the fixing roll is made higher, the gap produced between the heating and fixing roll 101 and the endless belt 115 becomes larger. This results from the thermal expansion of air or water vapor. That is, since the pressing force of the endless belt relative to the heating and fixing roll caused by belt tension is small, the gap is produced because the thermal expansion of air or water vapor cannot be suppressed.
Accordingly, the pressure roll and the pressure auxiliary roll have to apply a load enough to suppress the thermal expansion of air or water vapor not to produce a gap.
However, in a portion where the pressing force is small, that is, in the intermediary between the pressure roll 125 and the pressure auxiliary roll 129, a gap tends to be produced between the heating and fixing roll 101 and the endless belt 115 not to apply pressure enough to prevent the movement of the non-fixed toner 108. In the system which uses the heating and fixing roll and the pressing and fixing roll but does not use the endless belt, since pressure is high, such an inconvenience less occurs. However, in the belt nip system, since pressure is low, there is a great possibility in that an image of the toner 108 is disturbed by the foam. In the actual case, blur, deviation and disturbance of an image which are considered to be resulted from foam occur.